The use of organic trivalent phosphorus compounds, such as organic phosphites and phosphonites, for the stabilization of polymeric substrates subject to thermal, oxidative and/or actinic light induced degradation is known. U.S. Pat. Nos. 4,143,028; 4,318,845; 4,374,219; 4,524,166; 4,912,155 and 4,999,393 describe the use of seven- and eight-membered dibenzo[d,f][1,3,2]dioxaphosphepins and dibenzo[d,g][1,3,2]dioxaphosphocins as effective stabilizers for organic materials.
J. D. Spivack et al., Polymer Stabilization and Degradation, P. P. Klemchuk, editor, American Chemical Society, Washington, D.C., 1985, 247-257, describe the use of hydrolytically-stable seven- and eight-membered dibenzo[d,f][1,3,2]dioxaphosphepins and dibenzo[d,g][1,3,2]dioxaphosphocins, respectively, as processing stabilizers. The detailed synthesis, spectroscopic data and structural conformation of seven- and eight-membered dibenzo[d,f][1,3,2]dioxaphosphepins and dibenzo[d,g][1,3,2]dioxaphosphocins are described by S. D. Pastor et al. in Phosphorus and Sulfur, 15, 9 and 253 (1983); J. Heterocyclic Chem., 20, 1311 (1983); Phosphorus and Sulfur, 19, 1 and 285 (1984), Helv. Chim. Acta, 76, 900 (1993) and U.S. Pat. No. 5,292,785.
U.S. Pat. No. 4,481,317 discloses the synthesis of higher molecular weight hindered bisphenol diphosphonites and their use as stabilizers for polyolefins for protection against ultraviolet light and as protection for the high temperature processing of the polymer.
Japanese Sho 46/2902 reports that low molecular weight acyclic pyrophosphites are known and are useful as process stabilizers-for PVC and PVC/ABS copolymers. Neither the cyclic pyrophosphites nor their use as stabilizers for organic materials subject to oxidative, thermal or actinic light induced degradation is described or suggested by the prior art.